Valid 211300Z – 221200Z
…NO SEVERE THUNDERSTORM AREAS FORECAST…
Thunderstorms are possible from the southwestern deserts to the
lower Ohio Valley and Mid-South region.
The dominant upper-air feature for this forecast will be a broad
cyclone — now evident in moisture-channel imagery over the Great
Basin, CA and Desert Southwest, and centered near the southern
border of NV/CA. The current mid/upper-level low is expected to
weaken as the cyclone shifts eastward through the period, under
gradually more influence from the northern-stream wave train.
Meanwhile, a vorticity lobe, associated with a basal shortwave
trough now over northern Baja, should eject northeastward,
contributing to the formation of a new 500-mb low by 12Z over
southeastern CO. The original low will become a basal shortwave
trough subservient to the new low, and located to its southwest
across central/northern AZ.
At the surface, 11Z analysis showed a low near LSE, with cold front
extending across central IA, southeastern KS, central/southwestern
OK, approaching LBB, and moving southward across east-central NM.
By 00Z, the low should cross northwestern Lake Huron, with cold
front across Lower MI, southeastern IL, northern AR, southeastern
OK, and north-central/west-central TX. By 12Z, the front should
reach northern NY, central PA, northeastern KY, near MEM, through a
weak frontal wave near TXK, and across central/southwest TX.
…Southern Plains and vicinity…
A couple areas of widely scattered to scattered convection and
embedded thunderstorms appear most probable this afternoon into the
overnight period, both largely post-frontal:
1. Southern to eastern NM and TX South Plains this afternoon and
evening, in the zone of strongest cooling aloft and modest elevated
moisture above the frontal wedge, and
2. North-central TX and southern OK near the Red River,
extending/expanding northeastward late this afternoon into evening
over eastern OK and western/northern AR.
A few strong storms may be embedded within the convective area
shifting from northeast TX and southeastern/eastern OK into western
AR, with sporadic small hail and gusty winds possible. The bulk of
convection may be northeast of the previous MRGL area and also
behind the front. Though warm-sector moisture will be seasonally
favorable, with surface dew points commonly in the 60s F, weak
low-level lapse rates are expected amidst extensive low-cloud cover
and occasional precip. Meanwhile, low/middle-level lapse rates
above the boundary layer — observed to be weak at 12Z in OUN/FWD
soundings, may steepen but only slightly and gradually, given the
absence of substantial large-scale support ahead of the cold front.
This casts considerable doubt on strength of lift along/ahead of the
front at scales needed to develop/maintain severe convection,
despite the presence of a somewhat favorable parameter space with
increasing deep shear (potentially 40-50-kt effective-shear
magnitudes just prior to frontal passage) and pockets of 500-1500
J/kg MLCAPE. Any convection developing along or just ahead of the
boundary is likely to mature only gradually, given the modest lapse
rates, and also, to be undercut by the front.